The invention relates to a process and apparatus for correcting a pre-controlled reference flux for a field-oriented control, comprising a reference current model and/or a reference voltage model and a converter-powered multi-phase machine with a speed detection device.
In the journal "Automatisierungstechnische Praxis atp," 29th year, Issue 2, 1987, pages 83 to 88, a converter-powered multi-pole machine, in particular rotary current asynchronous machine with a field-oriented control, is shown, where the regulation contains a reference current model and a reference voltage model. With this drive, the entire speed setting range is divided into three segments as seen in FIG. 1. Starting from therefor a zero speed 7 to a nominal speed n.sub.n, constant torque M is required in the basic speed range. Correspondingly, the output P increase proportionally to the speed n, and constant values result for the flux-forming current component I.sub.1, since the machine is operated at a nominal flux .psi..sub.n. In the field weakening range I (speed range n.sub.n to 2n.sub.n), the voltage U is raised from a nominal value U.sub.n to its maximum value only at a constant output P, proportional to .sqroot.n, which corresponds to a reduction of the magnetization in inverse proportion to .sqroot.n. The moment-forming current component I.sub.2 decreases in the same manner. At values larger than twice the nominal speed n.sub.n in the field weakening range II, the voltage U remains constant at the output of the converter. Its value lies approximately 3% below the maximum possible output voltage of the converter at full block operation, and at a predetermined intermediate circuit voltage, in order to maintain slight peak currents for the inverter. With a constant output P, the moment-forming current component I.sub.2 does not change its value, while the flux-forming current component decreases in inverse proportion to the speed n. With this drive design, a sufficient distance from the overload torque must be maintained in the field weakening range, from the moment M. This is achieved by a low-leakage machine design and by the voltage increase.
If the multi-phase machine is put under greater stress at high speeds, the field-oriented control generates a reference voltage as the setting of the converter. This reference voltage is greater than the maximum converter output voltage, which can be generated as a function of an intermediate circuit voltage and at full block operation. In other words the regulation generates a setting for the converter, which the converter cannot follow since a corresponding output voltage would have to be greater than the maximum converter output voltage. Therefore, the actual flux no longer agrees with the reference flux. This causes departure from the field-oriented range and results in incorrect orientation.
If a cascade current control is used in the regulation concept, additional problems arise. Due to the incorrect flux setting, an incorrect magnetization current setting results, since the flux-forming current component I.sub.1 is equal to the quotient of the flux .psi. and the main field inductance L.sub.n of the machine. To impress this reference current, a converter voltage is required which cannot be attained by the intermediate circuit. This puts the current circuit out of operation, and, incorrect orientation occurs again.
There is a need for a method and apparatus that corrects pre-controlled reference flux for a field-oriented control and prevents incorrect orientation of the field-oriented control.